Stone's Cocoon(石の繭)

The body of a person who died an unnatural death is discovered encased in mortar like concrete in the basement of an abandoned building. Why was the person murdered this way? The next day, the investigation task force receives a phone call from the criminal who calls himself Tremi during a meeting to discuss the investigation of the case. Then, in a short time after her transfer to the First Investigative Division, Kisaragi Toko becomes the negotiator. Tremi mocks and provokes the police while offering hints regarding the murder. A second murder soon occurs just as Tremi had warned. The victim's head has been covered with concrete as they had expected. What is the criminal's motive and why is the criminal fixated with concrete? The criminal's surprising actual target soon emerges...